Lately I've been somewhat intimidated by the amount of stuff that looks set to happen in my current WIP. It got me thinking about complexity creep in an ongoing series, and how a writer can know if there's just too much going on.
I have my external conflict for Nathan to face; a new villain to take on, as well as the internal conflict he has to overcome by the time the book ends; finding a proper place for himself following the events of the first book. There are also new revelations as part of the ongoing storyline, showing more of the world Nathan has entered and the role he has played in it in past lives. So I have my three core plot elements. It's just the dressing that's making me squirm.
As it stands, I have major and minor characters from book 1 returning, as well as maybe half a dozen new characters of varying degrees of importance. I know that not all of the characters from book 1 need to return. In fact one won't be featured as more than a phone conversation until the next book beyond this, and some won't even return until book 4 in the series (really hope I'm not too presumptuous in planning ahead this much).
I think the key to returning characters is ensuring that, if they're there featured for more than just keeping the plot moving by providing assistance, or to serve as a simple obstacle, then they really should show something new of themselves and contribute to the hero's journey on a personal level. The same should go for new characters. They're either there to service the plot or there to help the protagonist grow.
I'm hoping that's what the other plot elements will do as well. This is still Nathan's story, after all, and while other characters can share his experiences and the themes can relate to them, it's Nathan's show. His continuing development needs to be what keeps the reader coming back. Of course, that's tough too since he hasn't quite yet become the hero he's meant to be. He's still got some hard lessons coming.
Where book 1 was about the sacrifices people make, book 2 is really about loss, and the acceptance of that loss, turning pain into the desire to become something better than you are right now.
I like to plan ahead. I have Nathan's story planned out in loose, changeable terms. It's going to be a hell of a ride. I just need to make sure I don't saturate the reader with too much all at once.